The invention relates to a digital pulse compression apparatus used in the pulse compression radar.
A conventional digital pulse compression apparatus is disclosed, for example, in the book "Radar Handbook", M. Skolnik, 2nd edition, pp, 10.8. The circuit block diagram of such a conventional apparatus is shown in FIG. 7. In FIG. 7, 1 is a phase detector, 2 is a COHO (Coherent Oscillator) generator, 3 is an A/D converter, 4 is a Fast Fourier Transform (FFT) circuit, 5 is a complex multiplier, 6 is a compression filter coefficient memory, 7 is an Inverse Fast Fourier Transform (IFFT) circuit and 8 is an amplitude detector.
The operation of the conventional digital pulse compression apparatus is hereinafter described in FIG. 7. The received IF (Intermediate Frequency) signal of the radar is detected in the phase detector 1 using the reference signal from the COHO generator 2 and is changed to I and Q vector video analog signals. The I and Q vector video analog signals are converted to I and Q vector video digital signals by the A/D converter 3, and the converted digital signals are inputted to the FFT circuit 4. The I and Q vector video signals are generally referred to as expanded pulses in the pulse compression radar, and are comparatively long pulses, which are modulated so that the auto-correlation functions are like impulses. Therefore, after the signal is converted by the fourier transform to a frequency spectrum X(.omega.) in FFT circuit 4, the output frequency spectrum X(.omega.) is multiplied in the complex multiplier 5 by the complex conjugate X*(.omega.) which is pre-stored in the compression filter coefficient memory 6. The resultant X*(.omega.).multidot.X(.omega.) is converted by inverse-fourier transform in the IFFT circuit 7 and returned to the time domain. At a result, a compression wave close to the impulse wave can be obtained. The amplitude information of the compression signal is obtained in the amplitude detector 8, and the amplitude information is used for the target detection. The contents of the above compression filter coefficient memory 6 may be substituted by X*(.omega.).multidot.W(.omega.) instead of the complex conjugate X*(.omega.) of the expanded pulse, where W(.omega.) is a weighting function, in order to suppress the range side lobe.
The received signal of the radar is shifted by the doppler effect in response to the moving of the target. Assume a doppler frequency .omega..sub.d, then the output of the FFT circuit 4 becomes X(.omega.+.omega..sub.d). The doppler effects of the output X(.omega.+.omega..sub.d) is generally well known as an ambiguity function which is disclosed, for example, in the book, David K. Barton, General Editor, "THE ARTECH RADAR LIBRARY", Raytheon Company, Vol III, Pulse Compression, pp. 124.about.132, C. E. Cook, M. Bernfeld and C. A. Palmieri, "Matched filtering, Pulse Compression and Waveform Design". That is, if the received signal includes the doppler frequency, the amplitude of the compression wave is decreased and the range side lobe is increased. The result badly influences the target detection, though the degree of influence is different depending upon the modulation type of the expanded pulse. If the pulse width of the expanded pulse is longer, the performance degradation becomes larger.